comparative developmental psychology: how is human ...comparative developmental psychology...

Evolutionary Psychology

www.epjournal.net – 2014. 12(2): 448-473

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Original Article

Comparative Developmental Psychology: How is Human Cognitive

Development Unique?

Alexandra G. Rosati, Department of Psychology, Yale University, New Haven, CT, USA. Email:

[email protected] (Corresponding author).

Victoria Wobber, Department of Psychology, Harvard University, Cambridge, MA, USA.

Kelly Hughes, Department of Brain and Cognitive Sciences, University of Rochester, Rochester, NY, USA.

Laurie R. Santos, Department of Psychology, Yale University, New Haven, CT, USA.

Abstract: The fields of developmental and comparative psychology both seek to illuminate

the roots of adult cognitive systems. Developmental studies target the emergence of adult

cognitive systems over ontogenetic time, whereas comparative studies investigate the

origins of human cognition in our evolutionary history. Despite the long tradition of

research in both of these areas, little work has examined the intersection of the two: the

study of cognitive development in a comparative perspective. In the current article, we

review recent work using this comparative developmental approach to study non-human

primate cognition. We argue that comparative data on the pace and pattern of cognitive

development across species can address major theoretical questions in both psychology and

biology. In particular, such integrative research will allow stronger biological inferences

about the function of developmental change, and will be critical in addressing how humans

come to acquire species-unique cognitive abilities.

Keywords: comparative development, primate cognition, heterochrony, human adaptation

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Introduction

Human behavior is strikingly different from that of other animals. We speak

language, routinely cooperate with others to reach complex shared goals, engage in abstract

reasoning and scientific inquiry, and learn a rich set of sophisticated cultural behaviors

from others. Why do we possess such abilities while other species do not? How do we

acquire these abilities? One approach in answering these core questions has been to

examine the roots of human cognitive systems. The field of developmental psychology

mailto:[email protected]

Comparative developmental psychology

Evolutionary Psychology – ISSN 1474-7049 – Volume 12(2). 2014. -449-

investigates the origins of adult cognition through the study of infants and children,

assessing how cognitive capacities emerge across ontogeny. This research has revealed that

human cognitive development is shaped by capacities thought to be derived in our species,

such as language and shared intentionality (Hermer-Vazquez, Moffet, and Munkholm,

2001; Pyers, Shusterman, Senghas, Spelke, and Emmoret, 2010; Tomasello, 1999;

Tomasello, Carpenter, Call, Behne, and Moll, 2005). The field of comparative psychology,

in contrast, investigates the evolutionary origins of human cognition via research with

humans’ closest living relatives, non-human primates, as well as other species of

evolutionary interest. This field has shown that many primate species exhibit an array of

complex abilities previously thought to be unique to humans, including reasoning about

others’ perceptions and knowledge, episodic memory and planning, and the ability to use

and create complex tools (Call, 2008; Flombaum and Santos, 2005; Humle and Matsuzawa,

2002; Martin-Ordas, Haun, Colmenares, and Call, 2009; Mulcahy and Call, 2006; Seed and

Byrne, 2010)

Both developmental and comparative approaches have therefore been critical in

generating and testing hypotheses regarding the origins of human-unique cognition. Yet

little attention has been paid to the intersection of developmental and comparative inquiry:

studies of cognitive development across species. Though several researchers have

highlighted the importance of a comparative developmental perspective (Gomez, 2005;

Langer, 2000; Lickliter, 2000; Matsuzawa, 2007; Matsuzawa, Tomonaga, and Tanaka,

2006), this integrative comparative-developmental approach has not been widely applied in

empirical work. Therefore, our current aim is to highlight the types of advances that

comparative developmental research can provide. In particular, we aim to show that

comparative developmental studies can contribute important insights into the origins of

human cognition by illuminating the pace and pattern of cognitive development across

species. We begin by examining theoretical models of how cognition can change across

species via changes in the pace and pattern of development. We then outline how studies of

comparative cognitive development can bear on current open questions in both psychology

and biology. Subsequently, we review empirical work incorporating comparative

developmental data in three cognitive domains: 1) reasoning about objects, 2) cognitive

skills used in foraging contexts, and 3) social cognition. Finally, we conclude by

identifying unsolved questions that can be addressed using a comparative developmental

framework. Overall, we argue that a comparative developmental approach is necessary to

understand the origins of human cognition, including features of development that are

unique to our species.

Changes in the pace and pattern of development

We propose that evolutionary changes in development can alter cognitive abilities

across species via two main mechanisms: differences in the pace or timing of cognitive

development, and differences in the pattern or structure of cognitive development. In terms

of change in pace, much research in developmental psychology is concerned with the

timing of skill emergence across human ontogeny. Methodological advances over the past

30 years have allowed scientists to probe the minds of even the youngest babies, showing

that infants have a sophisticated set of abilities for perceiving, categorizing, and reasoning



about social agents (Gergely, Bekkering, and Kiraly, 2002; Gergely, Nadasdy, Csibra, and

Biro, 1995; Woodward, 1998; Woodward, Sommerville, and Guajardo, 2001), the physical

world (Newcombe and Huttenlocher, 2005; Spelke, Breinlinger, Macomber, and Jacobson,

1992; Spelke, Lee, and Izard, 2010), and even abstract concepts such as number

(Feigenson, Dehaene, and Spelke, 2004; Wynn, 1998). Yet whereas some skills emerge

early in development, others have a much longer developmental trajectory. For example,

whereas younger children rely primarily on geometric information to reorient in an

environment, older children construct novel, flexible representations of space that combine

both geometric and spatial information (Hermer and Spelke, 1994; Hermer-Vazquez et al.,

2001; Hermer-Vazquez, Spelke, and Katnelson, 1999; Lee, Shusterman, and Spelke, 2006;

Lee and Spelke, 2010; Wang, Hermer, and Spelke, 1999). That is, different cognitive skills

can develop at different paces over ontogeny.

Figure 1: Differences in pace of development across species

Notes: In (a), Species X and Species Y initiate development with the same level of cognitive skill, but the

faster rate of skill acquisition in Species X results in a higher mature level of the cognitive skill compared

to Species Y. In (b), both species exhibit similar cognitive developmental trajectories, but because the onset

of development is delayed for Species Y, its mature cognitive skill level is lower than Species X. A similar

outcome results if the temporal onset of development occurs at a similar age, but the initial skill level at

that onset differs between species. Finally, in (c) both species begin development at a similar age and

exhibit similar trajectories, but because the developmental period is longer for Species X than Species Y,

Species X exhibits a higher level of the skill.

Despite extensive studies documenting the timing of cognitive development in

humans, little is known about how human pacing compares to that of other animals.

Understanding developmental pacing across species can illuminate differences in

developmental start-points, or the earliest-emerging skills, as well as differences in the

means by which later-emerging skills arise over development. Studies of evolutionary

changes in developmental timing, or heterochrony, suggest that there are three broad

categories of change in developmental pacing that can impact the expression of cognitive

skills in mature individuals (Gould, 1977; Lieberman, Carlo, Leon, and Zollikofer, 2007;

Wobber, Wrangham, and Hare, 2010a). First, two species could differ in their rate of skill

acquisition (Figure 1a), developing a skill for a comparable period of time across the

lifespan, but at differing speeds. Second, two species could differ in their start point of

development (Figure 1b), either because one species exhibits developmental changes in a

skill at an earlier age than the other, or because one species already possesses a higher level

of skill at the time when developmental change begins. Finally, two species could differ in



the age or life stage at which cognitive development concludes (Figure 1c). That is, one

species could exhibit a longer period of development compared to another. It is important

to note that these possibilities are not mutually exclusive. Consequently, two species could

differ across all of these features, or none of them.

The second major mechanism by which developmental changes could impact

cognitive skills across species concerns the pattern of development, or differences in how

development unfolds. In particular, a number of theories in psychology propose that

specific skills are essential prerequisites for the acquisition of other skills, therefore

requiring a specific pattern of development to acquire the mature adult skill level. For

example, in the area of theory of mind, young children have been found to exhibit a

relatively consistent pattern of success across different types of problems: children first

begin to understand that other people can have conflicting desires, then distinguish between

others’ knowledge states, and finally understand that others may have different (or even

false) beliefs about the world (Wellman and Liu, 2004, although see Onishi and

Baillargeon, 2005). Similarly, in the physical domain, human children’s capacities to track

objects have been found to progress in a consistent pattern as well: infants first represent

hidden objects, then overcome the A-not-B error, and finally master more complex

relationships such as invisible displacement (Piaget, 1954). Some of these theories suggest

that earlier-emegring abilities provide a developmental scaffold for later-emerging abilities.

However, it is currently unclear if these same patterns of development are shared across

other species.

Figure 2: Differences in the patterns of development across species

Notes: In (a), Species X and Species Y differ in their developmental sequence. Although both species

acquire the same set of skills, skill B is a prerequisite for C in Species X, whereas the reverse is true in

Species Y. In (b), the two species exhibit different causal structures of the development of skills that

emerge in the same order. In species X, skills B and C are dependent on the acquisition of skill A,

whereas in Species Y, skills B and C develop independently from skill A. Finally, in (c), Species Y

exhibits a terminal addition: The acquisition of a later-emerging skill (D) is dependent on the emergence

of skills A through C. However, this final skill does not emerge in Species X.

There are three broad categories in which patterns of cognitive development could

differ across species. First, two species could differ in their order of skill acquisition



(Figure 2a). Here, early-emerging skills in one species could differ from the early-emerging

skills in another. Second, two species could differ in the causal structure of skill

acquisition, even if the general order of skills remains similar (Figure 2b). That is, later-

emerging skills could be causally dependent on the acquisition of earlier-emerging skills in

one species, whereas these causal links could be missing in the comparison species. Finally,

some species could have terminal additions to their development, acquiring skills that are

not seen in other species (Figure 2c). Even if developmental patterns are similar in earlier

phases of development, one species might acquire skills that never arise in other species.

Insights from comparative development for psychology

The models described above indicate that studies of comparative development can

test psychological theories of human development. For example, some nativist theories of

human development suggest that young infants possess a core set of fundamental, evolved

cognitive capacities very early in life (e.g., Kinzler and Spelke, 2007). Others take a more

constructivist stance towards conceptual change, emphasizing the importance of learning-

based changes (Carey, 1985; Gopnik and Meltzoff, 1997; Tenenbaum, Griffiths, and Kemp,

2006; Wellman and Gelman, 1992). Finally, developmental systems theories focus more on

the developmental process itself—that is, how the mechanisms supporting development

evolve and change (Gottlieb, 1976, 1991; Lickliter, 2000). Importantly, these theories of

human cognitive development make divergent predictions concerning the pace and pattern

of development in non-human species.

The core knowledge theory, for example, suggests that some of the earliest-

emerging skills in human infants reflect cognitive abilities that do not change significantly

across development (Kinzler and Spelke, 2007). This view argues that a small set of

domain-specific, experience-independent capacities forms the basis of our earliest

understanding of the world, providing a scaffold on which later learning can build. Later in

development, language may allow children to integrate old information in new ways

(Hermer-Vazquez et al., 1999, 2001; Pyers et al., 2010). Within our comparative

framework, early similarities in core knowledge across species would predict that human

and non-human infants should possess a similar set of domain-specific precocious

capacities in the first few months of life, exhibiting comparable pace and pattern in

development during this period. Later in development, however, there should be more

marked divergences between the human and non-human pace and pattern of development

(coinciding with emerging language skills). These language-dependent shifts in human

cognitive development could occur via a prolonged developmental time course following

language emergence, or the terminal addition of novel skills (see Figures 1c and 2c).

Studies of non-linguistic species can therefore target the degree to which language is

necessary for certain ontogenetic changes to occur, versus whether language facilitates the

emergence of some skills without being strictly necessary, by examining if or when non-

humans acquire those skills.

Comparative developmental data can also test constructivist views of cognitive

development, which hypothesize that cognitive skills may be fundamentally transformed

across ontogeny. These types of views suggest that infants undergo a process of conceptual

change throughout development such that their “theories” about the world are reorganized,



(Carey, 1985; Gopnik and Meltzoff, 1997; Wellman and Gelman, 1992). Recent advances

suggest that these changes may occur via Bayesian learning, whereby infants update their

probability estimates for a hypothesis as they acquire additional evidence (Tenenbaum et

al., 2006). Since Bayesian mechanisms support some types of learning across many species

(Gold and Shadlen, 2007; Ma, Beck, Latham, and Pouget, 2006), the constructivist view

would imply that species differences in cognition might arise from differences in initial

start points (see Figure 1b) or in the total length of the developmental process (see Figure

1c), rather than via differences in the rate or pattern of learning throughout development, as

suggested by nativist or core knowledge theories.

Insights from comparative development for biology

Studies of comparative cognitive development can also provide important insights

for biology. The comparative method has long been one of the most important tools in

evolutionary biology for reconstructing historical processes that often cannot be directly

observed, such as evolution (Harvey and Purvis, 1991; Mayr, 1982). This method contrasts

traits across species (including cognitive abilities; MacLean et al., 2012) that have faced

differing ecological or social pressures in order to understand how natural selection

influenced the manifestation of those traits (Harvey and Purvis, 1991; Mayr, 1982). As

such, comparative biology can address not just what differences exist between species (e.g.,

the phylogeny of traits), but also why those differences arose (the evolutionary function of

traits; Tinbergen, 1963).

Studies of comparative development have become increasingly important in the

field of evolutionary developmental biology, which has revealed that species differences in

mature traits frequently arise through alterations in developmental trajectories (Gould,

1977). In particular, novel traits are not typically created de novo. Rather, evolutionary

shifts in the pattern and pace of developmental timing can generate variation among

individuals and, ultimately, between species. One example of this process is the origin of

variation in beak morphology across finch species living in the Galapagos Islands. As

initially documented by Charles Darwin (1854), finches in this archipelago possess beaks

of differing length and breadth, allowing different species to feed on different types of

food. Recent research has revealed that these differences stem from variation in the

expression over development of a gene controlling calcium production during beak growth

(Abzhanov et al., 2006). That is, changes in the developmental timing of this (shared) gene

generate diversity in adult beak morphology.

Importantly, this same approach used to examine morphological differences can be

applied to cognitive or behavioral capacities. The human intellect has long been recognized

as one of our species’ most striking characteristics, creating an evolutionary puzzle for

theorists since Darwin (1871). Comparative developmental studies provide a novel means

to understand how the human mind emerged, and in particular to assess whether differences

in human cognition stem from evolutionary changes in our species’ developmental

trajectories. More broadly, comparative studies of cognitive development allow complex

behavioral patterns to be broken down into their constituent cognitive processes, which

may be dissociable in terms of their differing developmental patterns. Thus, this approach

facilitates evolutionary analyses of cognition as an important biological trait that also



exhibits variation across species, much like morphological traits that have been the focus of

previous developmental work.

Empirical Approaches to Comparative Developmental Psychology

The previous sections highlighted how the comparative developmental approach

can address outstanding questions in psychology and biology, and proposed theoretical

models of how variation in the pace and pattern of development can produce variation in

cognitive abilities across species—including humans. In the current section, we apply these

models to three empirical domains that have been the focus of recent research on

comparative development: 1) reasoning about objects, 2) foraging cognition, and 3) social

cognition.

Development of object reasoning in primates

Since Piaget’s (1954) early work examining how human children come to

understand that objects continue to exist when they are out of sight (e.g., object

permanence), children’s development of physical concepts has been an area of active

research and debate. Some researchers claim that infants only acquire rich representations

of objects by interacting with the physical world (Smith and Gasser, 2005), whereas others

suggest that, in the absence of direct evidence, even very young infants are equipped with

sophisticated abilities to represent objects (Feigenson et al., 2004; Spelke et al., 1992).

Comparative developmental studies with other animals—whose interactions with the

physical world have both similarities and differences from those seen in humans—can

therefore address the role of experience in generating children’s mature abilities to reason

about objects.

One of the first developmental studies of physical reasoning examined object

classification (Langer, 1980, 1986). This work identified a basic developmental pattern

underlying infants’ object classification in spontaneous play behavior. During their first

year, infants play at creating sets of objects, and eventually form sets that share similar

properties. In the second year of life, children construct object sets with complex

properties, and in the third year they begin to relate objects physically—for example, by

making buildings and bridges. Finally, in their fourth year, children play with multiple

object sets simultaneously and show mutability of sets, such as first sorting by color and

then later by material (Langer, 1980, 1986). These studies indicate that spontaneous play in

humans appears to have a relatively fixed pattern of development, with certain skills (such

as sorting sets) emerging reliably before other skills (such as encoding multiple sets).

Does object play reflect a human-specific developmental trajectory, or do these

patterns reflect a deeply rooted cognitive structure that other primates share? Studies of

object play in several species, including chimpanzees (Pan troglodytes), capuchins (Cebus

apella), and longtailed macaques (Macaca fascicularis), in fact suggest that their basic

order of skill acquisition in object play is similar to that of human infants, even though

these species lack much of the experience with objects that human infants have (Hayashi

and Matsuzawa, 2003; Poti, 1997; Poti, Hayashi, and Matsuzawa, 2009; Spinozzi, 1983;

Spinozzi and Natale, 1989). However, there are also important differences from humans.



For example, monkeys of all ages played with set creation, but did not consistently create

similarity sets until 2 to 3 years of age. Development was quicker in chimpanzees, but still

slower than in humans, and the apes never showed the most complex forms of object

constructions. Overall, these results suggest that apes (including humans) exhibit a faster

pace of development than do monkeys in skills of object manipulation (as in Figure 1a).

Nonetheless, the most complex skills that humans exhibit may be terminal additions in

human development (as in Figure 2c).

Another important skill for reasoning about the physical world involves tracking the

locations of objects as they move through space. In humans, object-tracking skills emerge

in a distinct order. First, infants first begin to retrieve hidden objects (before 10 months of

age), and then begin to comprehend visible displacements, solving the “A not B” error,

around their second birthday (Call, 2001; Piaget, 1954). Soon after, they solve invisible

displacements, but it is not until relatively late in development (3 to 4 years) that children

understand complex invisible displacements such as transpositions (changing the location

of an object’s container) and rotational displacements (where the support is moved; Piaget,

1954; see also Call, 2001; Lasky, Romano, and Wenters, 1980; Okamoto-Barth and Call,

2008; Sophian, 1984). One hypothesis for this consistent pattern of emergence is that all

object-tracking skills rely on the same set of cognitive capacities. As these capacities

develop, children can complete more complex tasks, such as those involving out-of-sight

movements or larger numbers of items (Barth and Call, 2006; Call, 2003). If this is the

case, then non-human primates should exhibit similar developmental patterns.

Alternatively, these behaviors may rely on a set of independent skills, in which case the

patterns of emergence might differ across species.

Some data from adult primates support the claim that these diverse object-tracking

behaviors rely on the same cognitive capacities. In particular, both monkeys and apes are

successful at “easier” tasks involving visible displacements, but generally only apes are

capable of the full range of object-tracking skills, including those with invisible movements

(Albiach-Serrano, Call, and Barth, 2010; Barth and Call, 2006; Beran, Beran, and Menzel,

2005; Beran and Minahan, 2000; Call, 2001; Collier-Baker, Davis, Nielsen, and

Suddendorf, 2006; de Blois, Novak, and Bond, 1998, 1999; Tinklepaugh, 1928; but see

Filion, Washburn, and Gulledge, 1996; Mendes and Huber, 2004; Neiworth et al., 2003).

However, other evidence from adult primates does not accord with the patterns seen in

humans. For example, even though adult monkeys are not successful at reasoning about

invisible displacement tasks, they can understand rotational displacements, which is

surprising because this requires mental reconstruction of the movements of hidden objects

(Hughes, Mullo, and Santos, 2013; Hughes and Santos, 2012; Poti, 2000). Furthermore,

adult apes show similar levels of errors on tasks that seem to differ in their difficulty for

human children, such as rotational displacements and translocations (Albiach-Serrano et

al., 2010) or transpositions and invisible displacements (Barth and Call, 2006). These

findings therefore cast doubt on the hypothesis that the same core skill set underlies

performance across these tasks in non-human primates.

Comparative developmental studies also suggest that object-tracking tasks tap into a

range of abilities that differ in their time and pattern of emergence. The pattern of success

across development by non-human primates in object-tracking tasks broadly mirrors that



seen in humans, but appears to proceed at a faster pace at early stages (as in Figure 1a; see

also Gomez, 2005). However, for a few select tasks, non-humans show different patterns

from those seen in humans, both in terms of the order of emergence (as in Figure 2a) and

the causal structure of skills (as in Figure 2b). In particular, the ability to follow

translocations develops at a later life-stage in non-human primates than in humans. For

example, gorillas first solve invisible displacements by 12 months (Dore and Goulet, 1998),

yet they learn to solve translocations sometime after 17 months (Visalberghi, 1986)—the

opposite order than that observed in children. Similarly, although human children tend to

solve “single” and “double” displacement tasks (which differ in the number of objects that

move) around the same time (Call, 2001; Piaget, 1954), double displacements appear to be

markedly more difficult for young non-human primates (Antinucci, 1990). Indeed, one

young gorilla solved invisible displacements many months before he was able to

understand a double visible displacement (Antinucci, Spinozzi, and Natale, 1986; Dore and

Goulet, 1998). Overall, these developmental data indicate that object-tracking abilities do

not develop in a uniformly consistent manner across species. Consequently, object tracking

may indeed depend on multiple underlying cognitive capacities in human development.

Comparative developmental inquiry, therefore, provided a key test for theories that could

not be fully evaluated with human developmental data alone.

Development of foraging skills in chimpanzees and bonobos

A second area where comparative developmental research has recently addressed

the origins of human cognition concerns cognitive skills used in foraging contexts. Wild

primates face a myriad of complex spatial problems when foraging, including recalling the

location of resources and determining the best route to navigate between these different

locations. Despite the ubiquity of such spatial problems, psychological theories suggest that

humans are uniquely equipped to solve spatial problems with a degree of flexibility and

accuracy not seen in other species. In particular, language is thought to play an important

role in human spatial cognitive development (Haun, Rapold, Call, Janzen, and Levinson,

2006; Hermer and Spelke, 1994; Hermer-Vazquez et al., 1999, 2001; Levinson, Kita, Haun,

and Rasch, 2002; Pyers et al., 2010). In terms of spatial memory, children exhibit

developmental shifts around 2 years of age in many skills, including recall of the locations

of hidden targets, recall of multiple locations, and recall of locations over longer temporal

periods (Balcomb, Newcombe, and Ferrara, 2011; Newcombe and Huttenlocher, 2005;

Newcombe, Huttenlocher, Drummey, and Wiley, 1998; Sluzenski, Newcombe, and Satlow,

2004). Some aspects of these changes may be related to language. For example, when

children are asked to search for hidden objects in a large space after they have been

disoriented, their proficiency with spatial language predicts the success of their searches

(Balcomb et al., 2011). However, the relationship between language and the emergence of

a given cognitive ability could be causal (language is required for a skill to emerge),

facultative (language speeds up a process that could occur otherwise), or only correlated.

Studies of comparative development in apes can therefore be an important tool in

differentiating these possibilities, as apes share an extended juvenile period of post-natal

brain development with humans but lack language (Matsuzawa, 2007; Matsuzawa et al.,

2006).



Increasingly, research indicates that apes have quite sophisticated spatial abilities

(Kuhlmeier and Boysen, 2002; Mendes and Call, 2008; Menzel, Savage-Rumbaugh, and

Menzel, 2002; Menzel, 1973). Studies tracking the emergence of spatial skills over

development further suggest that apes can show human-like changes in spatial abilities in

the absence of language. For example, one study examined how chimpanzees and bonobos

performed in a naturalistic foraging task where they could search for food they had seen

hidden previously in a large outdoor enclosure (Rosati and Hare, 2012b). Whereas infant

apes found few pieces of food, older chimpanzees were more successful at recalling

multiple locations. Thus, although there are improvements in place memory associated with

the acquisition of spatial prepositions in humans (Balcomb et al., 2011), chimpanzees

exhibit similar developmental changes without any concurrent language acquisition. This

suggests that at least some types of ontogenetic shifts in place-based searching may be due

to intrinsic, maturational changes in spatial abilities. In addition, although language may be

a developmental prerequisite for the origins of some spatial skills in humans (Hermer-

Vazquez et al., 2001), this causal link is not present in apes (following Figure 2b).

Comparative developmental studies also make it possible to address why such

differences in developmental trajectories arise, by linking developmental data with

information about different species’ natural histories. For example, in the foraging task

described previously (Rosati and Hare, 2012b), chimpanzees and bonobos exhibited similar

skills in infancy, but chimpanzees exhibited a faster rate of improvement than bonobos

(following Figure 1a). Why might this be the case from an evolutionary perspective?

Importantly, even though chimpanzees and bonobos diverged less than 1mya (Won and

Hey, 2005), they show a suite of differences in their behavior and morphology.

Chimpanzees exhibit more pronounced sexual dimorphism, increased escalated aggression,

and higher rates of extractive foraging and hunting, whereas bonobos exhibit increased

sociosexual behavior and an increased importance of female bonds (Hare, Wobber, and

Wrangham, 2012; Kano, 1992; Parish, 1996; Parish and de Waal, 2000; Wrangham and

Pilbeam, 2001). One hypothesis for this pattern is that chimpanzees and bonobos differ in

their wild feeding ecology (Kano, 1992; Wrangham and Peterson, 1996). In particular,

chimpanzees are thought to live in environments where they face higher costs to acquire

food, including more seasonal variation (White, 1998), more competition for smaller, less-

abundant food patches (White and Wrangham, 1988), and have less access to important

fallback foods (Malenky and Wrangham, 1993)—a set of differences that is thought to

impact the tenor of social interactions in this species. This ecological hypothesis further

suggests that chimpanzees develop more accurate spatial memory over ontogeny due to

their greater dependence on patchily-distributed resources as adults.

Comparisons of chimpanzees and bonobos therefore provide a test case for the role

of comparative developmental psychology in illuminating the functional relevance of

cognitive capacities. Increasing evidence suggests that adult chimpanzees and bonobos

actually differ in a set of cognitive traits related to foraging, including levels of feeding

tolerance (Hare, Melis, Woods, Hastings, and Wrangham, 2007; Wobber, Wrangham, and

Hare, 2010b), tool use abilities (Herrmann, Hare, Call, and Tomasello, 2010), spatial

memory (Rosati and Hare, 2012b), and patterns of value-based decision-making

(Heilbronner, Rosati, Hare, and Hauser, 2008; Rosati and Hare, 2011, 2012a, 2013; Rosati,



Stevens, Hare, and Hauser, 2007). One potential evolutionary mechanism to generate these

differences is heterochronic changes in dvelopmental timing. Indeed, developmental

comparisons have revealed that even though patterns of cognitive development are broadly

similar for both species, bonobos appear to exhibit delayed development specifically in this

set of skills relevant to the ecology hypothesis, such as in inhibitory control and memory

(Herrmann, Hare, et al., 2010; Rosati and Hare, 2012b; Wobber, Herrmann, Hare,

Wrangham, and Tomasello, 2013; Wobber et al., 2010b). That is, chimpanzees and

bonobos exhibit targeted differences in development only for a specific set of ecologically-

relevant skills.

Comparative developmental studies of closely related species (like chimpanzees

and bonobos) can allow researchers to integrate hypotheses about the mechanisms

supporting cognition with hypotheses about the functional importance of those cognitive

skills in natural behavior. Studies that further include both humans and non-human apes

can therefore provide critical insights into the evolutionary function of uniquely-human

cognitive skills. For example, human feeding ecology differs from that of other great apes

in several ways. Hunter-gatherers have larger home ranges than other apes, and exhibit a

unique pattern of central place foraging where individuals return to a centralized location

with food (Hill, Barton, and Hurtado, 2009; Marlowe, 2005). Consequently, humans are

more reliant on distant food sources than are other apes, which poses new problems

concerning locating food and navigating between resources. The results from chimpanzees

and bonobos predict that humans may also exhibit targeted developmental differences in

evolutionarily-relevant skills. For example, over development, children form increasing

flexible representation of space integrating multiple cues (Hermer-Vazquez et al., 1999,

2001; Newcombe and Huttenlocher, 2005), which may be related to the new ecological

problems that humans must solve. Comparative developmental studies can potentially

illuminate the function of these human cognitive abilities.

Development of social cognition in apes and humans

A final area where recent comparative developmental work has advanced our

knowledge is that of social cognition. Humans and non-human primates differ not only in

how they reason about the physical world, but also in their reasoning about the social

world. Humans possess a sophisticated belief-desire psychology that we use to reason

about the (unobservable) thoughts, intentions, and beliefs of other people. Other primates

share some of the same capacities, such as reasoning about other’s perceptions, but they

also differ in important ways (Call, 2008; Rosati, Santos, and Hare, 2010; Wobber and

Hare, in press). In fact, one influential hypothesis suggests that social cognition represents

the most profound difference between us and other animals (Tomasello, 1999; Tomasello

and Carpenter, 2007; Tomasello et al., 2005). Recent studies have begun to disentangle

how these differences emerge.

First, humans and other primates exhibit many striking similarities in their social

cognition in the first few months of life. Chimpanzee and rhesus macaque neonates, for

example, both imitate the facial expressions of a human demonstrator in their first few days

of life (Ferrari et al., 2006; Myowa-Yamakoshi, Tomonaga, Tanaka, and Matsuzawa,

2004), much like babies (Meltzoff and Moore, 1983). In addition, infant chimpanzees and



an infant gibbon were found to prefer faces with direct gaze over averted-gaze faces by the

age of 2 months (Myowa-Yamakoshi and Tomonaga, 2001b; Myowa-Yamakoshi,

Tomonaga, Tanaka, and Matsuzawa, 2003), paralleling the “2-month revolution” when

humans infants begin to actively engage with other’s gaze (Rochat, 2001). Finally, infant

chimpanzees and an infant gibbon recognize and prefer the face of their mother or human

caregiver over less familiar faces at 1 month of age (Myowa-Yamakoshi and Tomonaga,

2001a; Tomonaga et al., 2004), much like human infants (Pascalis, Deschonen, Morton,

Deruelle, and Fabregrenet, 1995). Taken together, these results show that humans and apes

exhibit similar developmental trajectories in early infancy, suggesting that human patterns

have deep biological roots.

However, human and non-human primate social development diverges at older

ages. For example, human infants reliably follow other’s gaze to targets by 6 months

(Butterworth and Jarrett, 1991) and begin to follow gaze in more complex situations, where

barriers make it difficult to gauge another’s line of sight, by 12 months (Moll and

Tomasello, 2004). In contrast, a longitudinal study of a chimpanzee’s gaze-following skills

indicates that he did not follow a human experimenter’s gaze until 13 months (Okamoto et

al., 2002). Other studies find that chimpanzees are not proficient in geometric gaze-

following or following gaze around barriers until 2 to 3 years old (Okamoto-Barth,

Tomonaga, Tanaka, and Matsuzawa, 2008; Tomasello and Carpenter, 2005; Wobber et al.,

2013). These results indicate that humans show an accelerated pace of social-cognitive

development in this period of late infancy (following Figure 1a). Macaques also develop

gaze-following skills at a slower rate than humans (Ferrari, Coude, Gallese, and Fogassi,

2008; Ferrari, Kohler, Fogassi, and Gallese, 2000; Teufel, Gutmann, Pirow, and Fischer,

2010), leading to the proposal that non-human gaze-following is spurred by experience

with relevant social interactions over a long critical period. This proposal would therefore

suggest a difference between humans and other primates, both in the rate of development

(see Figure 1a) as well as a difference in the underlying causal structure of gaze-following

skills (see Figure 2b). Indeed, mechanisms supporting gaze-following in adult chimpanzees

and humans may differ as well. Eye-tracking studies show that although humans are most

interested in looking at eyes, chimpanzees are as interested (or even more interested) in

viewing the bodies or mouths of conspecifics (Kano and Tomanaga, 2009; Myowa-

Yamakoshi, Scola, and Hirata, 2012).

Another area of divergence in human and non-human social cognitive development

concerns the perception of goal-directed action. Human infants rapidly gain proficiency in

understanding goal-directed actions: They interpret the acquisition of desirable objects as

being the goal of a reaching gesture by 6 months, and reason about the rationality of an

experimenter’s goal-directed action by 14 months (Gergely et al., 2002; Woodward, 1998).

In contrast, chimpanzees only first begin to reason about others’ actions in terms of their

underlying goals around 2 to 3 years of age, even when reared by humans (Bering,

Bjorklund, and Ragan, 2000; Tomasello and Carpenter, 2005; Wobber et al., 2013). Thus,

both gaze following and goal understanding appear to emerge relatively late in

chimpanzees compared to humans. This again supports the claim that there are differences

in both the pace (see Figure 1a) and pattern (see Figure 2b) of human social cognitive

development, suggesting that behavioral skills that quickly come online in relatively young



humans may depend on additional social experience in non-humans.

Earlier differences in human and primate social cognition may have further

cascading effects over development. Human infants typically rely on mechanisms of joint

attention to comprehend and direct others’ attention and behavior following the “9-month

revolution.” At this point, infants first begin to move beyond simple dyadic interactions

(self and other) to engage triadic interactions (self, other, and referent; Carpenter, Nagell,

and Tomasello, 1998; Tomasello, 1999). In contrast, chimpanzee infants do not seem to

exhibit joint attention, even though they can understand and even direct others’ behavior at

older ages (Tomonaga, 2006; Tomonaga et al., 2004; Wobber et al., 2013). Overall, these

findings indicate that although joint attention may be a causal precursor of other skills in

humans (such as more complex forms of gaze following, as described previously), this

causal structure is lacking in apes (as in Figure 2b). These results highlight that even

though certain capacities may be prerequisites for later-developing abilities in humans,

these same capacities may not be tightly linked in other species.

Indeed, even enculturated apes—who have been reared in a human-like social and

communicative environment—do not exhibit capacities for joint attention. Enculturation

studies can assess the degree to which ape developmental trajectories are plastic in

response to human-like social inputs. Enculturated apes can use some symbolic language,

employ referential signals outside of the linguistic domain (Premack and Premack, 1983;

Terrrace, 1979), imitate others (Bjorklund, Bering, and Ragan, 2000; Call and Tomasello,

1996), and even reason about novel tools like humans (Furlong, Boose, and Boysen, 2008).

However, they do not develop human-like joint attention (Carpenter, Tomasello, and

Savage-Rumbaugh, 1995; Tomasello and Carpenter, 2005; but see Bard et al., 2005 for

conflicting definitions of joint attention). These findings suggest that important differences

between human and ape social cognition persist even when apes are reared in more human-

typical environments.

Finally, the importance of these early differences in social-cognitive development

may increase in later ontogeny. Tomasello and colleagues hypothesized that the early

emergence of social cognitive skills like joint attention and goal understanding allows

humans to rapidly acquire a diverse array of skills across domains (Herrmann, Call,

Hernadez-Lloreda, Hare, and Tomasello, 2007; Tomasello and Carpenter, 2007; Tomasello

et al., 2005). In particular, skills emerging during the “9-month revolution” (Tomasello,

1999), such as the ability to engage in triadic interactions, appear to enable human infants

to learn proficiently from other individuals—including social knowledge such as novel

word labels (Baldwin and Moses, 2001; Vaish, Demir, and Baldwin, 2011), as well as non-

social knowledge about how to correctly utilize novel tools (Birch, Vauthier, and Bloom,

2008; Casler and Kelemen, 2005). This hypothesis predicts that because non-humans do

not acquire this earlier set of social skills, they will begin to exhibit divergences in both

social and non-social skills in later ontogeny.

A recent comparison of cognitive development in 2- to 4-year-old humans,

chimpanzees, and bonobos assessed this hypothesis by comparing the performance of these

species across a diverse array of cognitive tasks. This comparison found that in the social

domain (in tasks assessing goal understanding, gaze-following, and imitation, among other

capacities), human 2-year-olds were more proficient than same-age non-human apes.



However, humans, chimpanzees, and bonobos had comparable performance at this age in

tasks measuring cognitive skills in the physical domain (such as assessing comprehension

of space, number, and causality). The critical test of the hypothesis was thus to examine

humans’ and apes’ relative change in skills as they aged. In support of the claim that early

social differences have cascading effects, humans showed greater improvement in their

performance on both social and physical tasks compared to apes (Wobber et al., 2013).

These results suggest that children’s early competency in social cognitive skills such as

goal understanding and imitation might enable them to learn more rapidly across domains,

whereas young apes must rely to a greater degree on individual and trial-and-error learning,

acquiring knowledge about the world at a slower rate. That is, the causal relationships

between the acquisition of different types of skills differs between apes and humans (as in

Figure 2b) may in turn facilitate differences in the rate of development later in ontogeny (as

in Figure 1a). Concordant with this suggestion, a study of individual differences in

performance across a similar battery of tasks found that children possess a distinct social

cognition “factor” predictive of their success in tasks across that domain, whereas

chimpanzees’ success on different social cognition tasks was not interrelated (Herrmann,

Hernández-Lloreda, Call, Hare, and Tomasello, 2010). Taken together, these results

indicate that human social-cognitive development differs from that of our closest relatives

in both pace and pattern. Moreover, these early differences in social abilities may underpin

major differences in the adult cognition of these species.

Conclusions: The Future of Comparative Developmental Psychology

We have argued that comparative developmental research can provide new insights

into human cognition that cannot be identified via developmental or comparative research

alone. Although this approach is not commonly used, recent research has already begun to

identify similarities and differences in the pace and pattern of human development relative

to other primates in domains as diverse as reasoning about objects, spatial memory, and

social cognition. Yet this empirical work represents a small sample of the open questions in

psychology and biology that the comparative developmental approach can address. Indeed,

there are some cognitive domains where it is already clear that comparative developmental

studies can distinguish between alternative accounts of cognitive development and move

theoretical debates forward.

For example, one current debate concerns the development of different capacities

supporting mature theory of mind. As mentioned previously, results from theory of mind

tasks suggest that these skills emerge in a strict order, with children first understanding

others’ desires, then their knowledge, and finally their beliefs (Wellman and Liu, 2004).

However, recent work using looking-time measures to test younger children find that

babies may understand something about false beliefs within the second year of life

(Baillargeon, Scott, and He, 2010; Kovács, Téglás, and Endress, 2010; Onishi and

Baillargeon, 2005; Southgate, Senju, and Csibra, 2007; Surian, Caldi, and Sperber, 2007).

Comparative studies of non-human primates further indicate that species such as

chimpanzees and rhesus macaques exhibit a number of theory of mind skills (see Rosati et

al., 2010 for a review). These skills include visual and auditory perspective-taking



(Braeuer, Call, and Tomasello, 2007; Flombaum and Santos, 2005; Hare, Call, Agnetta,

and Tomasello, 2000; Hare, Call, and Tomasello, 2006; Melis, Call, and Tomasello, 2006;

Santos, Nissen, and Ferrugia, 2006), reasoning about others’ knowledge states (Hare, Call,

and Tomasello, 2001; Kaminski, Call, and Tomasello, 2009; Marticorena, Ruiz, Mukerji,

Goddu, and Santos, 2011), and desire comprehension (Buttelmann, Call, and Tomasello,

2009). Critically, however, non-human primates do not appear to represent the beliefs of

others when tested in false belief tasks (Call and Tomasello, 1999; Kaminski et al., 2009;

Krachun, Carpenter, Call, and Tomasello, 2009; Marticorena et al., 2011), and may instead

represent situations in which an individual has a false belief merely in terms of that

individual’s ignorance (Kaminski et al., 2009; Marticorena et al., 2011). Comparative

developmental studies can thus assess whether the patterns of emergence of different theory

of mind capacities in non-human primates are in fact causally dependent on one another,

and illuminate why young infants might be successful on non-verbal false-belief tasks

whereas non-humans are not. Alternatively, non-human primates’ capacities may emerge

through different developmental mechanisms, suggesting that their abilities are supported

by different representations about social agents.

A second debate that comparative developmental studies can address concerns the

development of our capacity to individuate objects. Over the past decade, developmental

researchers have examined how infants develop the ability to distinguish objects of

different categories. Classic work by Xu and colleagues suggested that emerging language

capacities—specifically, learning the labels of different objects—may be necessary for

conceptually individuating different kinds of objects (Xu, 2002; Xu and Carey, 1996; Xu,

Carey, and Quint, 2004; Xu, Carey, and Welch, 1999). Indeed, Xu and colleagues observed

that infants are unable to individuate two different objects in a looking-time task before

they know the words for those objects (Xu and Carey, 1996). To test the role of language,

primate researchers have begun to examine whether adult non-human primates can succeed

on similar individuation tasks. Surprisingly, this work revealed that adult primates are able

to individuate objects, even though they lack their linguistic labels (Munakata, Santos,

Spelke, Hauser, and O’Reilly, 2001; Phillips and Santos, 2007; Santos, Sulkowski,

Spaepen, and Hauser, 2002). How then do non-human primates learn to solve individuation

problems that human infants solve with language? Comparative developmental studies can

address this topic by focusing on the emergence of these mechanisms in non-human

primate ontogeny. For example, monkeys may acquire object individuation skills at a

slower pace than humans, suggesting that language may play a facultative role in

accelerating human development of these skills. Comparative developmental studies can

also identify whether primates’ individuation capacities depend on another form of

experience that operates similarly to the way human language facilitates individuation in

our own species.

Finally, a major question in biology that comparative developmental studies can

address concerns the evolutionary significance of differences in human life history traits

(i.e., the timing and patterning of key life events such as age of sexual maturity and

reproduction). Whereas all primates exhibit a long juvenile period relative to other

mammals (Charnov and Berrigan, 1993), the human juvenile period is markedly longer

than expected relative to other primates (Bogin and Smith, 1996; Walker, Burger, Wagner,



and Von Rueden, 2006). One hypothesis for the evolutionary function of this prolonged

juvenile period is that it enables more complex skill acquisition and, therefore, greater

behavioral flexibility in human adulthood (Bjorklund and Green, 1992; Bogin, 2010;

Kaplan, Hill, Lancaster, and Hurtado, 2000). Yet although there is morphological and

physiological evidence indicating that human growth and reproduction is slowed relative to

that of other primates, there are little data on comparative cognitive development

examining whether humans actually exhibit longer periods of skill acquisition. Indeed,

some evidence comparing rates of cognitive development in humans and apes suggests that

human cognitive development may in fact be accelerated in comparison to that of non-

human primates during early childhood, rather than prolonged (Wobber et al., 2013).

Comparative developmental studies of apes and humans at older ages—such as during

adolescence, when humans are learning some of their most complex, culturally-acquired

skills—can further test this hypothesis.

Overall, we emphasize that studies of comparative cognitive development can

address outstanding questions in both psychology and biology. Research using this

approach has already made important contributions to our understanding of the human

mind. Indeed, the results discussed here highlight that models of human cognitive evolution

must integrate data on cognitive development in other species in order to draw strong

inferences about what traits are unique to our lineage. Thus, the comparative developmental

approach represents an untapped line of evidence that will advance theoretical ideas

concerning what cognitive abilities are unique to humans, as well as the ultimate

evolutionary function of these abilities in human evolutionary history.

Acknowledgements: These ideas were developed at an International Conference for Infant

Studies (2012) symposium, and we thank attendees and commentators. A.G.R.’s research is

supported by a grant from the L.S.B Leakey Foundation. V.W.’s research is supported by

grants from the L.S.B. Leakey Foundation, NSF-BCS-0851291 (DDIG), and the Wenner-

Gren Foundation. L.R.S. is supported by a McDonnell Scholar Grant (#220020242).

Received 10 September 2012; Revision submitted 16 May 2013; Revision submitted 08

October 2013; Accepted 09 October 2013

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